I've got some rather nice Recaro seats and have been looking for mountings / slides.

Checking the seats out in the car shows that I sure as hell don't need them raised! I'm looking at the Recaro Universal double locking slides at £70 odd but without coughing up for them I can't see if they are long enough front to back to meet up with the under-floor (chassis) brackets.. looks like they are too short.

What has anyone else used for seat mounts/slides? Maybe I get the Recaros and weld (or bolt) them to a a flat steel section 25 x 400 or so to allow bolting to the chassis brackets? Any thoughts?

The mounts on the chassis are Recaro spacing, I use the sliders you talk about on mine and they're spot on. They also only raise the seat by about 1", you don't want anymore than that.
You might get a slightly cheaper solution from car builder solutions, but check the bolt spacing.
You can also get a bracket from Recaro that will sit between the runner and seat and give you a mount for the buckle part of the seat belt, well worth having so the buckle moves with the seat and you don't need to have those incredible ugly flexible buckles.